Synthesis of indolizidines related to the carditonic pumiliotoxin a alkaloids

ABSTRACT

A method of making dendrobatid alkaloids with the formula ##STR1## in which A is either CH 3  or H, and n is an integer from 1-9, and R is CH 2  OH, CHO, ##STR2## and R 1  and R 2  are alkyl groups; and the use of such alkaloids to treat cardiovascular illness.

This invention was made with government support under Grant No. HL25854awarded by the National Institute of Health. The government has certainrights in this invention.

FIELD OF INVENTION

This invention relates to indolizidine chemical intermediates, methodsof preparing the same, and uses of the intermediates to preparedendrobatid alkaloids.

BACKGROUND OF THE INVENTION

The skin of the Central American poison frogs, Dendrobates pumilio,contains a variety of alkaloids, some of which are known cardioactiveagents as described by Witkop in The Alkaloids, Page 200, Volume 24(Eds. Arnold and Brossi). Unfortunately, however, these chemicals havenot been employed for medical treatment because they are not practicallyobtainable from natural sources. One group, consisting of two structuraldistinct subgroups, termed pumiliotoxins A and B was discovered as earlyas 1967 and with their allo equivalents includes at least 24 members.While structural studies have shown that these toxins share the unusual(Z)-6-alkylideneindolizidine ring system which carries side chains atpositions C-6 and C-8 with the C-6 chain being connected to the ring byan exocyclic double bond, there have been no reports detailing methodswhereby pumiliotoxins A or B can be made, and only one describing thesynthesis of a simple pumiliotoxin A alkaloid 251D. Pharmacologicalstudies have shown that pumiliotoxin B is a considerably more potentcardioactive drug than pumiliotoxin 251D. Thus, a method which wouldpermit the practical synthesis of pumiliotoxin B and related dendrobatidalkaloids would facilitate their use in the treatment of cardiovasculardisorders.

Although there hve been no previous reports on the synthesis ofpumiliotoxin B, two related but structurally distinct pumiliotoxin Aalkaloids, specifically pumiliotoxins 251D and 237A have recently beensynthesized. Neither the synthesis of pumiliotoxins 237A nor 251D,however, involve the use of the key indolizidine alcohol and aldehydeintermediates which is the subject matter of the present invention.Moreover, neither synthesis employs the use of the ylide used in thepresent invention to generate pumilitoxin B.

As stated previously, there is at present no known synthetic schemewhereby pumiliotoxins B can be generated. However, a recent report bythe present inventor has described a method that is useful for thestereo controlled synthesis of the side chain connected to the(Z)-6-alkylideneindolizidine ring system, that is, a method forassembling the allyic diol functionality of pumiliotoxin B. This workwas done on model chemicals that do not contain an indolizidine ring,specifically (2S,4E)-heptenones, and these chemicals have no knowncardioactive property.

SUMMARY OF THE INVENTION

According to the present indolizidine invention, alcohol and aldehydeintermediates are synthesized wherein the intermediates are generatedfrom a reaction of trialkylsilyl-ω-primary ethers with oxiranes, andsubsequent reaction steps. The key indolizidine aldehyde intermediatesupon subsequent reaction with suitable ylides yield pumiliotoxin Bdendrobatid alkaloids.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows silylalkyne, 2, used in the synthesis of indolizidineintermediates.

FIG. 2 depicts the synthetic scheme for the indolizidine alcohol andaldehyde intermediates.

FIG. 3 shows the synthesis of ylide, 11.

FIG. 4 depicts the preparation of pumiliotoxins B.

DETAILED DESCRIPTION OF INVENTION

The invention described herein relates to chemical intermediates thatare useful to synthesize cardioactive drugs. Accordingly, we haveproduced novel alcohol and aldehyde indolizidine intermediates that arepractically useful in the synthesis of dendrobatid alkaloids. The novelaldehyde intermediate is generated from the alcohol intermediate and theformer is reacted with a novel ylide to make dendrobatid alkaloids.

(A) Synthesis of the key alcohol and aldehyde indolizidine intermediates

To generate indolizidine alcohol and aldehyde intermediates useful inthe synthesis of pumiliotoxin B, it is necessary to assemble a chiralsilylalkyne from esters of(S)-5-(benzyloxy)-1-trimethyl-silyl-1-pentyn-3-ol. This involves thereduction of 3-(benzyloxy)propanitrile, the latter reagent beingprepared from acrylonitrile and benzyl alcohol as described by Gaiffeand Launay in Academy of Science, Serial C, Pages 1379-1380 (1968), withi-Bu₂ AlH to give 3-benzyloxy propanal which is condensed withethynyllithium, prepared as described by Midland in the Journal ofOrganic Chemistry, Volume 40, Page 2250 (1975), to yield the racemicpropargylic alcohol in about 50% yield following distillation. Reactionof the propargylic alcohol with R-(+)-2-methylbenzylamine gives thecorresponding diastereomeric carbamates that upon chromatographicseparation and carbamate cleavage with Cl₃ SiH as described by Pirkle inthe Journal of Organic Chemistry, Volume 42, Page 2781 (1971) yields the(S)-alcohol and (R)-alcohol derivatives in yields of about 20%.

Isolation of the (S)-alcohol and subsequent carbon silylation andacetylation yields the (S)-acetate in about 50% yield. The corresponding(S)-benzoate can be prepared from the (R)-alcohol similarly by carbonsilylation followed by Mitsunobu inversion with benzoic acid. Either the(S)-acetate or the (S)-benzoate upon reaction with the Me₂ CuMgBr in THFgives the sought after silylalkyne in about 50% yields from eitherreagent.

Next, silylalkyne is sequentially reacted at 22° C. with i-Bu₂ AlH andMeLi, followed by reaction with the appropriate chiral epoxide generatedfrom L-proline as described by Overman in the Journal of the AmericanChemical Society, Volume 103, Pages 1851 (1981) to yield thecorresponding bicyclic carbamate. Base hydrolysis of the latter at 90°C. followed by reaction of the amino alcohol at room temperature withaqueous formalin yields a cyclopentaoxazolidine which is cyclized to thedesired Z-alkylideneindolizidine by reaction with camphorsulfonic acidin refluxing acetonitrile. Chromatographic purification gives theZ-alkylideneindolizidine in about 50% yield. Debenzylation of theZ-alkylideneindolizidine by conventional methods yields the key alcoholintermediate which upon subsequent oxidation with the Swern reagent bythe general procedure described by Swern in the Journal of OrganicChemistry, Volume 43, Page 2480 (1978) provides the importantindolizidine aldehyde intermediate.

(B) Synthesis of Ylide:

In order to obtain the proper stereochemistry of the side chain allylicdiol of pumiliotoxins B, it is necessary to prepare a ylide withsuitable stereochemistry. This is accomplished by inversion of ethylL-lactate by the general procedure of Mitsunobu described in Synthesis,Volume 1, Page 1 (1981) at -20₀ C.° in THF to give the corresponding(R)-p-nitrobenzoyl ester. The latter reagent is deacylated with K₂ CO₃in anhydrous ethanol and the alcohol produced protected using thegeneral technique of Hanessian and Lavallee described in the CanadianJournal of Chemistry, Volume 55, Page 562 (1978) as thet-butyldiphenylsilyl ether. Finally, ylide is generated by way of the2-pyridinethiol ester as described by Overman and McCreedy inTetrahedron, Volume 23, Page 2355 (1982).

(C) Reaction of indolizidine aldehyde with ylide:

To generate pumiliotoxin B reaction of indolizidine aldehydeintermediate with ylide is effected by refluxing in CH₂ Cl₂ to yield thecorresponding enone that upon threo selective reduction with LiAlH₄ andaccompanying desilylation gives pumiliotoxin B. Subsequentchromatographic purification yields, in about 95% isomerically pureform, pumiliotoxin B.

EXAMPLE I Preparation of(8S,8aS)-8-Hydroxy-8-methyl-6Z-[4-oxo-2R-methylbutylidene]octahydroindolizidine(1)

In order to make the key intermediate(8S,8aS)-8-Hydroxy-8-Methyl-6Z-[4-oxo-2R-methylbutylidene]octrahydroindolizidine(1) the following reactions are carried out as diagramed in FIGS. 1 and2. The numbers that refer to reagents in the text below correspond toreagents in the diagram.

(a) (R)-5-(Benzyloxy)-3-Methyl-1-(trimethylsilyl)pentyne (2) (FIG. 1)

Using the general procedure of McDonald, described in the Journal ofOrganic Chemistry, Volume 45, Page 4740 (1980), a solution of the(S)-acetate 3 (4.50 g, 14.8 mmol) and THF (15 mL) was added at 23° C.dropwise to the cuprate reagent prepared in THF (75 mL) from CuI (2.81g, 29.6 mmol) and MeMgBr (31 mL) of a 1.9M solution in ether). After 10min at 23° C., the reaction was quenched by adding it dropwise to anexcess of NH₄ Cl (saturated aq). Isolation (ether, MgSO₄) andchromatographic purification (20:1 hexane ethyl acetate) gave 1.98 g(51%) of 2 as colorless liquid: 98.7% pure by GLC analysis; [a]_(D) ²⁵-64.0 (c 2.26, CHCl₃); IR (film) 2167, 1262, 1100 cm⁻¹ ; ¹ H NMR (250MHz, CDCl₃) 7.2-7.4 (m, Ph), 4.52 (s, OCH₂ Ph), 3.61 (apparent t, J=6.5Hz, CH₂ CH₂ O), 2.6-2.75 (m, C.tbd.CCH), 1.65-1.80 (m), 1.18 (d, J=7.0Hz, Me), 0.13 (s, Me₃ Si); MS (isobutane CI) m/z 261 (MH), 233, 187,171, 129, 103, 91, 79, 73; MS (EI) m/z 245.1368 (245.1362 calcd for C₁₅H₂₁ OSi, M-Me).

An identical procedure can also be used to prepare 2 from benzoate 4(3.98 g, 10.8 mmol): yield 1.23 g (44%); [a]_(D) ²⁵ -64.4 (c 1.42,CHCl₃).

(b) Synthesis of(1S,7aS)-Tetrahydro-1-methyl-1-[6-(benzyloxy)-4R-methyl-2-(trimethylsilyl)-2Z-hexenyl]-1H,3H-pyrrolo[1,2-c]oxazol-3-one(5) (FIG. 2)

Neat i-Bu₂ AlH (0.85 mL, 4.8 mmol) was added dropwise at 23° C. to asolution of 2 (1.23 g, 4.74 mmol) and hexane (3 mL). After 0.5 h THF (8mL) was added followed by the dropwise addition of MeLi (2.6 mL of a1.33M solution in ether, 3.5 mmol). The color changed from colorless tolight pink when the last drop of MeLi was added, and slowly changed over0.5 h to light yellow as this solution was maintained at 23° C. Asolution of epoxide 6 (520 mg, 1.99 mmol made as described by Overman inthe Journal of the American Chemical Society, Volume 103, Page 1851(1981) and THF (2 mL) was then added and the resulting solution washeated at 60° C. for 25 h. Workup gave a yellow oil which was purifiedby chromatography (4:1 hexane-ethyl acetate) to give 637 mg (77% basedon 12) of 5 as a colorless oil: [a]_(D) ²⁵ -39.2 (c 1.05, CHCl₃); IR(film) 1760 cm⁻¹ ; ¹ H NMR (250 MHz, CDCl₃) 7.2-7.4 (m, Ph), 5.87 (d,J=10.6 Hz, =CH), 4.48 (AB quartet, J=11.9 Hz, OCH₂ Ph), 3.53-3.66 (m,NCH and NCHH), 3.46 (apparent t, J=6.8 Hz, CH₂ O), 3.05-3.2 (m, NCHH),2.55-2.65 (m, ═CHCH), 2.56 (AB quartet, J=13.7 Hz, CH₂ C═), 1.5-2.2 (m),1.30 (s, C-8 Me), 0.99 (d, J=6.6 Hz, CHMe), 0.17 (s, Me₃ Si); MS(isobutane CI m/z 416 (MH), 372, 186, 139, 107, 91, 70; MS (EI) 400.2306(400.2307 calcd for C₂₃ H₃₄ NO₃ Si, M-Me).

(c)(8S,8aS)-8-Hydroxy-8-methyl-6Z-[4-(benzyloxy)-2R-methylbutylidene]octahydroinodolizidine(7) (FIG. 2)

A carefully degassed solution of 5 (527 mg. 1.27 mmol), KOH (1.8 g, 25mmol), EtOH (1.6 mL) and H₂ O (0.4 mL) was heated at 90° C. for 12 h.After cooling to 23° C., 37% aq formalin (1 mL) and MeOH (2 mL) wereadded dropwise. After 4 h the reaction was concentrated and the crudeoxazolidine 8 (496 mg) was isolated (CH₂ Cl₂, K₂ CO₃): MS (isobutane CIm/z 402 (MH).

A mixture of this oxazolidine sample, paraformaldehyde (180 mg, 6.0mmol), camphorsulfonic acid (650 mg, 2.8 mmol), and acetonitrile (10 mL)was heated at 80° C. for 13 h. Isolation (ether, K₂ CO₃) gave a brownoil which was purified by radial chromatography (silica gel, 50:1:0.1CHCl₃ -MeOH-12N NH₄ OH) to afford 219 mg (52%) of 7 as a light yellowoil: pure by TLC analysis; [a]_(D) ²³ -25.5 (c 2.33, CHCl₃); IR (film)3200-3600, 1100 cm⁻¹ ; ¹ H NMR (250 MHz, CDCl₃) 7.2-7.4 (m, Ph), 5.02(br d, J=9.7 Hz, ═CH), 4.45 (apparent s, OCH₂ Ph), 3.82 (d, J=11.8 Hz,H-5α), 2.29 (br d, J=11.8 Hz, H-5β), 1.13 (s, C-8 Me), 1.00 (d, J= 6.6Hz, C-11 Me); MS (isobutane CI) m/z 330, 166, 107, 91, 83, 70; MS (EI)m/z 329.2338 (329.2355 calcd for C₂₁ H₃₁ NO₂).

(d)(8S,8aS)-8-Hydroxy-8-methyl-6Z-[4-hydroxy-2R-methylbutylidene]octahydroindolizidine(9) (FIG. 2)

A solution of 7 (164 mg., 0.500 mmol), THF (8 mL) and NH₃ (8 mL) wastreated with excess Li until the blue color persisted for 3 h. Additionof excess NH₄ Cl was followed by evaporation of the NH₃ and isolationwith CHCl₃ (K₂ CO₃) to give, after radial chromatography (silica gel,20:1:0.1 CHCl₃ -MeOH-12N NH₄ OH), 73.1 mg (61%) of 9 as a colorless oil:isomerically pure by TLC analysis (R_(f) 0.4, C-11 epimer R_(f) 0.5;10:1.0:0.1 CHCl₃ -MeOH-12N NH₄ OH), [a]_(D) ²⁵ -22.1 (c 1.87, CHCl₃); IR(film) 3100-3600, 1670 cm⁻¹ ; ¹ H NMR (250 MHz, CDCl₃) 5.06 (br d, J=9.8Hz, ═CH), 3.82 (d, J=11.8 Hz, H-5α), 3.45-3.65 (m, CH₂ O), 3.0-3.12 (m,NCHH), 2.7 (br s, OH), 2.55-2.70 (m, ═CCH), 2.37 (br d, J=11.9 Hz, H-5β), 2.17-2.3 (m, NCHH), 2.14 (ABq, J=14.0 Hz, CH₂ C═), 1.5-2.3 (m), 1.14(s, C-8 Me), 1.02 (d, J=6.6 Hz, C-11 Me); MS (EI) m/z 239.2876 (239.1885calcd for C₁₄ H₂₅ NO₂).

This key alcohol indolizidine intermediate could be stored in a freezerunder Ar for several months with only slight decomposition.

(e)(8S,8aS)-8-Hydroxy-8-methyl-6Z-[4-oxo-2R-methylbutylidene]octahydroindolixidine(1)

Lastly, the formation of the key aldehyde intermediate 1 was made fromalcohol 9 (240 mg, 1.00 mmol) by oxidation with oxalyl chloride (3 mmol)and Me₂ SO (6 mmol) using the general procedure developed by Swern asdescribed in the Journal of Organic Chemistry, Volume 43, page 2480(1978), to give, after radial chromatography (silica gel, 25:1:0.1 CHCl₃-MeOh-12N NH₄ OH), 211 mg (88%) of 1 as a slightly impure pale yellowoil: [a]_(D) ²⁵ -25.9 (c 0.63, CHCl₃); IR (film) 3100-3700, 1718, 1030cm⁻¹ ; ¹ H NMR (250 MHz, CDCl₃) 9.67 (t, J=2.0 Hz, CHO); MS (isobutaneCI) m/z 238 (MH), 117, 99. 1 should be used immediately in theolefination step in the preparation of pumiliotoxins B.

EXAMPLE II Preparation of Pumiliotoxin B using indolizidine aldehydeintermediates

The intermediate(8S,8aS)-8-hydroxy-8-methyl-6Z-[4-oxo-2R-methylbutylidene]octhydroindolizidine(1) can be used to produce pumiliotoxin B (12) by carrying out thefollowing set of seguential reactions:

(a) Formation of(8S,8aS)-8-Hydroxy-8-methyl-6-Z[6-oxo-2R,5-dimethyl-7R-(t-butyldiphenylsilyoxy)-4E-octenylidene]octahydroindolizidine(10) (FIG. 4)

A carefully degassed solution of 1 (53.3 mg, 0.224 mmol), ylide 11 (194mg, 0.323 mmol), and CH₂ Cl₂ (1 mL) was heated at reflux for 5 d.Concentration and purification of the residue by radial chromatography(silica gel, 40:1:0.1 CHCl₃ -MeOH-NH₄ Oh) gave 136 mg of a light yellowoil which was a ˜2:1 mixture of 10 and Ph₃ PO. This mixture wasdifficult to separate and it was typically used directly in thereduction step, since Ph₃ PO and 12 are easy to separate bychromatography.

A 30 mg portion of this crude oil was purified by careful preparativeTLC (R_(f) 0.3, 40:1 CHCl₃ -MeOH, eluted twice), to give 19.3 mg (71%)of chromatographically homogeneous 10 as a colorless oil: [a]_(D) ²⁵+13.2 (c 0.87, MeOH); IR (film) 3100-3600, 1685, 1110 cm⁻¹ ; ¹ H NMR(250 MHz, CDCl₃) 7.2-7.7 (m, Ph), 6.19 (br t, J=7.3 Hz, CH═CCO), 5.01(br d, J=9.8 Hz, ═CH), 4.69 (q, J=6.8 Hz, CHOR), 3.74 (d, J=11.7 Hz,H-5α), 3.2-3.1 (m), 2.5-1.6 (m), 1.54 (br s, ═CMe), 1.24 (d, J=6.8 Hz,ROCHMe), 1.08 (s, C-8 Me), 1.00 (s, t-Bu), 0.89 (d, J=6.5 Hz, CHMe); MS(isobutane CI) m/z 560 (MH), 304, 279, 257, 231; MS (EI) m/z 559.3463(559.3480 calcd for C₃₅ H₄₉ NO₃ Si).

(b) Formation of (+)-Pumiliotoxin B (12) (FIG. 4)

A solution of pure enone 10 (14.6. mg, 0.026 mmol) and THF (0.5 mL) wasadded dropwise at -20° C. to a rapidly stirred suspension of LiAlH₄ (8mg, 0.2 mmol) and THF (1.5 mL). After 0.5 h, the cooling bath wasremoved and the mixture was allowed to warm to 23° C. After 2 h, Na₂SO₄.10H₂ O (0.1 g) was added followed by CHCl₃ (5 mL), and the resultingmixture was stirred rapidly for 2 h and then filtered through Celite.The concentrated filtrate was purified by radial chromatography (silicagel, 10:1:0.1 CHCl₃ -MeOH-12N NH₄ OH) to give 6.2 mg (74%) of 12 as acolorless oil. This sample was chromatographically homogeneous, butcontained 6% of the erythro diastereomer (¹ H NMR analysis of thep-bromophenylboronides). An analytical specimen of synthetic 12 wasobtained from the center cut of a chromatographic purification of alarger sample of comparable material: [a]_(D) ²⁵ +19.3, [a]₅₇₈ ²⁵ +19.7,[a]₅₄₆ ²⁵ +23.0, [a]₄₃₅ ²⁵ +40.2 (c 1.00, MeOH); IR (film) 3100-3600,1455, 1375, 1310, 1090, 1028, 966 cm⁻¹ ; ¹ H NMR (250 MHz, CDCl₃, 4.7mg/0.5 mL) 5.39 (br t, J=6.7 Hz, H-13), 5.06 (br d, J=9.6 Hz, H-10),3.7-3.9 (m, H₅α and H-16), 3.70 (apparent d, J=7.1 Hz, H-15), 3.0-3.1(m, H-3α) 2.66 (br s, OH), 2.42-2.57 (m, H-11), 2.34 (br d, J=11.7 Hz,H-5β), 1.65-2.32 (m), 1.58 (br s, H-19, 1.13 (s, H-9), 1.11 (d, J=6.0Hz, H-17), 1.00 (d, J=6.6 Hz, H-18); ¹³ C NMR (63 MHz, CDCl₃) 135.4,133.9, 130.7, 127.6, 82.9, 71.8, 69.0, 68.6, 54.7, 53.4, 49.0, 35.7,32.6, 24.5, 23.4, 21.5, 21.3, 19.1, 12.4, MS (EI) m/z 323.2466 (323.2460calcd for C₁₉ H₃₃ NO₃ 18%), 306 (10%), 278 (40%), 206 (23%), 194 (53%),193 (31%), 176 (16%), 166 (100%), 70 (98%).

EXAMPLE III Synthesis of Ylide (11) (FIG. 3)

The first step in the synthesis of Ylide 11 employed several stepsdescribed below:

(a) Formation of Ethyl 2-(R)-(4-Nitrobenzoyloxy)propionate (13)

A modification of Mitsunobu's procedure was employed wherein a solutionof Ph₃ P (3.14 g, 12 mmol) and THF was added dropwise at -20° C. to arapidly stirred solution of diethyl azodicarboxylate (2.0 mL, 12 mmol)and THF (2 mL). A white precipitate appeared within 0.5 h. A solution of4-nitrobenzoic acid (1.67 g, 10 mmol and THF (15 mL) was added dropwiseat -20° C., the resulting mixture was stirred at -20° C. for 0.5 h, andethyl L-lactate (13) (2.0 mL, 18 mmol, [a]²⁰ -12 (neat), AldrichChemical Co.) was then added by drops. The cooling bath was removed andthe reaction mixture was allowed to stir for 27 h. Concentration gave anorange oil which was diluted with ether and filtered to remove a whitesolid. Purification of the filtrate by chromatography (1:1 hexane-ethylacetate) gave 2.42 g (91%) of pure 14 as a low melting white solid: mp42°-43° C. (from hexane), [a]_(D) ²⁵ -13.1 (c 1.17, EtOH); IR (film)1720, 1430, 1280, 1110 cm⁻¹ ; ¹ H NMR (250 MHz, CDCl₃) 8.2-8.35 (m, arylH) 5.35 (q, J=7.0 Hz, OCH), 4.25 (q, J=7.1 Hz, OCH₂), 1.67 (d, J=7.0 Hz,OCHMe), 1.30 (t, J=7.1 Hz, CH₂ Me); MS (isobutane CI) m/z 268 (MH), 222,150, 120, 71; MS (EI) m/z 267.0742 (267.0743 calcd for C₁₂ H₁₃ NO₆).

(b) Formation of Ethyl 2(R)-(t-butylidiphenylsilyloxy)propanoate (15)

A mixture of 14 (1.66 g, 6.22 mmol), K₂ CO₃ (850 mg, 6.2 mmol, flamedried) and absolute EtOH (10 mL) was stirred at 23° C. for 15 min.Filtration and concentration of the filtrate gave crude ethyl D-lactate,which was immediately silylated at 23° C. (6 h) in DMF (10 mL) witht-butyldiphenylsilyl chloride (1.8 mL, 7.0 mmol) and imidazole (950 mg,14 mmol). Aqueous workup (ether, MgSO₄) gave a viscous oil which waschromatographed (7:1 hexane-ethyl acetate) to give 1.18 g (53%) of pure15 as a colorless liquid: [a]_(D) ²⁵ +46.0 (c 1.97, EtOH), IR (film)1762, 1110, 1140 cm⁻¹ ; ¹ H NMR (250 MHz, CDCl₃) 7.3-7.7 (m, Ph), 4.27(q, J=6.7 Hz, OCH), 4.02 (q, J=7.1 Hz, OCH₂), 1.37 (d, J=6.7 Hz, CHMe,1.14 (t, J=7.1 Hz, CH₂ Me), 1.10 (s, t-Bu); MS (EI) m/z 299.1104(299.1103 calcd for C₁₇ H₁₉ SiO₃, M-Bu, 72%), 227 (70%), 199 (100%).

(c) Formation of S-2'-Pyridinyl2-(R)-(t-butyldiphenylsilyloxy)propanthioate (16)

A solution of 15 (775 mg, 2.17 mmol), KOH (620 mg, 8.8 mmol), and MeOH(10 mL) was maintained at 23° C. for 10 h and then concentrated.Acidification with 1N HCl (20 mL), extraction with ether, and drying ofthe organic extract (MgSO₄) provided the corresponding crude acid (579mg, 82%) as a colorless liquid. This material was immediately esterifiedin ethyl acetate (10 mL) by reaction with 2-pyridinthiol (222 mg, 2.0mmol), and dicyclohexylcarbodiimide (474 mg, 2.33 mmol) at 23° C. for 9h. After separation of the precipitate by filtration, the filtrate waswashed with brine, dried (MgSO₄) and concentrated to give a yellow oil.Purification on silica gel (4:1 hexane-ethyl acetate) gave 620 mg (68%from 14 of 16 (purity ˜90% by ¹ H NMR analysis) as a colorless oil: IR(film) 1715 cm⁻¹ ; ¹ H NMR (250 MHz, CDCl₃) 8.6-8.7 (m, C-6 pyridinylH), 7.2-7.8 (m, pyridinyl H), 4.44 (q, J=6.6 Hz, OCH), 1.26 (d, J=6.6Hz, CHMe), 1.18 (s, t-Bu); MS (isobutane CI) 422 (MH), 279, 257, 225,112, 89.

(d) (Formation of Ylide 2-(R)-(t-butyldiphenylsilyoxy)-4-triphenylphosphoranylidene-3-pentanone (11)

A solution of s-BuLi (1.91 mL of a 1.15M solution in cyclohexane) wasadded at 23° C. to a rapidly stirred suspension of(ethyl)triphenylphosphonium bromide (816 mg, 2.20 mmol, finely groundand dried in vacuo for 2 d at 200° C.) and THF (15 mL). Over the periodof 10 min the white suspension changed to a dark red solution. Asolution of 16 (421 mg, 1.00 mmol) and THF (5 mL) was added by drops,the resulting orange suspension was stirred at 23° C. for 20 min, andwas then filtered through Celite. Agueous workup (ether, K₂ CO₃) gave690 mg of a yellow oil. This material was purified by radialchromatography (4 mm silica gel plate, 1:1 hexane-ethyl acetate) to give323 mg (54%) of ylide 11 as a nearly pure light yellow amorphous solid:IR (film) 1518, 1440, 1110 cm⁻¹ ; ¹ H NMR (250 MHz, CDCl₃) 7.3-7.8 (m,Ph), 4.56 (q, J=6.5 Hz, CHO), 1.58 (d, J=6.8 Hz, P═CMe), 1.32 (d, J=6.5Hz, CHMe), 1.08 (s, t-Bu); MS (CI) m/z 601 (MH), 317, 279, 263, 185, 85.

EXAMPLE IV Biological Effects of Synthetic Pumiliotoxin B

Synthetic pumiliotoxin B was tested for cardioactive properties onatrial strips prepared from male 250 gm guinea pigs by suspending thestrips in 20 ml of a physiologically balanced salt solution aerated witha 95:5 mixture of O₂ :CO₂ gas as described by Perry in PharmacologicalExperiments on Isolated preparations (2nd. Ed., Churchill Livingston,New York, 1970). The effect of synthetic pumiliotoxin B on the rate andforce of spontaneous contractions was examined by serial additions ofaliquots of the alkaloid in methanol as described by Mensah-Dwumah andDaly in Toxicon, Volume 16, Page 189 (1978). Synthetic pumiliotoxin Bincreases the force of contractures of spontaneously beating guinea pigatrial strips by 2 to 5 fold with half maximal effects at about 3 uM,and increases rates of atrial contractions by 2-3 fold with half maximaleffects at about 6 uM.

We claim:
 1. An indolizidine alkaloid with the formula ##STR3## wherein:(a) A is either CH₃ or H, and(b) n is an integer from 1-9, and (c) R isCH₂ OH, CHO, or ##STR4## and (d) R₁ and R₂ are alkyl groups both largerthan CH₃, or a combination of alkyl groups, one being CH₃ and the otherlarger than CH₃.
 2. The indolizidine alkaloid in claim 1 wherein:(a) Ais CH₃, n is 1 and R is CH₂ OH, or (b) A is CH₃, n is 1 and R is CHO, or(c) A is CH₃, n is 1 and R is ##STR5## and (d) R₁ and R₂ are alkylgroups both larger than CH₃, or a combination of alkyl groups, one beingCH₃ and the other larger than CH₃.
 3. The indolizidine alkaloid in claim1 wherein:(a) A is H, n is 1 and R is CH₂ OH, or (b) A is H, n is 1 andR is CHO, or (c) A is H, n is 1 and R is ##STR6## and (d) R₁ and R₂ arealkyl groups both larger than CH₃, or a combination of alkyl groups, onebeing CH₃ and the other larger than CH₃.
 4. A method of makingindolizidine alkaloid comprising the steps of:(a) reacting oneequivalent of a chiral protected cohydroxy silylalkyne with ahydroaluminating reagent, and less than one equivalent of MeLi in hexaneyielding a vinyl alanate, followed by reacting said vinyl alanate withan epoxide yielding a bicyclic carbamate, and (b) sequential treatmentof said bicyclic carbamate with hydroxyl ions effecting hydrolysis ofsaid bicyclic carbamate, and subsequent reaction with formalin andcyclization of the product with camphorsulfonic acid in acetonitrile,and (c) alcohol protection of the product.
 5. The method as defined inclaim 4 in which the alcohol protection of the product is oxidized. 6.The method as defined in claim 5 in which said oxidized product isreacted with an ylide and the product reduced.
 7. The method as definedin claim 4, wherein:(a) said chiral protected cohydroxy silylalkyne hasthe formula ##STR7## and (b) said epoxide is the chiral epoxide has theformula ##STR8## and (c) said indolizidine alkaloid is thez-alkylideneindolizidine has the formula ##STR9## and (d) R₄ and R₅ arealcohol protecting groups.
 8. A method as defined in claim 4 inwhich:(a) said chiral protected cohydroxy silylalkyne has the formula##STR10## and (b) said epoxile is the chiral epoxide has the formula##STR11## and (c) said indolizidine alkaloid is thez-alkylideneindolizidine has the formula ##STR12## and (d) R₄ and R₅ arealcohol protecting groups.
 9. The method as defined in claim 6 in whichsaid ylide has the formula:(a) ##STR13## and (b) R₁ and R₂ are CH₃ orCH₃ CH₂ and R₆ is an alcohol protecting group.
 10. A method forincreasing the force and rate of atrial tissue contractions comprisingcontacting indolizidine alkaloids with said atrial tissue in aqueoussolution containing 3-6 uM of said indolizidine alkaloids wherein saidindolizidine alkaloids have the structure ##STR14## and; (a) A is eitherCH₃ or H, and(b) n is an integer from 1-9, and (c) R is CH₂ OH, CHO, or##STR15## and (d) R₁ and R₂ are alkyl groups both larger than CH₃, or acombination of alkyl groups, one being CH₃ and the other larger thanCH₃.
 11. A method for increasing the force and rate of atrial tissuecontractions comprising contacting indolizidine alkaloids with saidatrial tissue in aqueous solution containing 3-6 uM of said indolizidinealkaloids wherein said indolizidine alkaloids have the structure##STR16## and (a) A is CH₃, n is 1 and R is CH₂ OH or(b) A is CH₃, n is1 and R is CHO, or (c) A is CH₃, n is 1 and R is ##STR17## and (d) R₁and R₂ are alkyl groups larger than CH₃, or a combination of alkylgroups, one being CH₃ and the other larger than CH₃.
 12. A method forincreasing the force and rate of atrial tissue contractions comprisingcontacting said indolizidine alkaloids with said atrial tissue inaqueous solution containing 3-6 uM of said indolizidine alkaloidswherein said indolizidine alkaloids have the structure ##STR18## and (a)A is H, n is 1 and R is CH₂ OH, or(b) A is H, n is 1 and R is CHO, or(c) A is H, n is 1 and R is ##STR19## and (d) R₁ and R₂ are alkyl groupslarger than CH₃, or a combination of alkyl groups, one being CH₃ and theother larger than CH₃.
 13. A method of synthesizing indolizidinealkaloid comprising the steps of:(a) reacting one equivalent of a chiralprotected silylalkyne at about room temperature with one equivalent of ahydroaluminating reagent and with about 0.85 equivalent of MeLi inhexane yielding a vinyl alanate; (b) reacting at about 60° C. said vinylalanate with about 0.44 equivalents of an epoxide having the structure##STR20## yielding a bicyclic carbamate having the structure ##STR21##(c) sequentially treating said bicyclic carbamate with hydroxyl ions atabout 90° C. to effect hydrolysis of said bicyclic carbamate, to give anintermediate that upon nitrogen deprotection is cyclizable yielding aZ-6-alkylideneindolizidine.
 14. A method as defined in claim 13 in whichthe protected product is oxidized.
 15. A method as defined in claim 14in which said oxydized product is reacted with a ylide comprising thestructure ##STR22## where R₁ and R₂ are CH₃ and R₆ is an alcoholprotecting group.
 16. A method for increasing the force and rate ofatrial tissue contractions comprising contacting substantially puresynthetic indolizidine alkaloids with said atrial tissue in aqueoussolution containing 3-6 uM of said indolizidine alkaloids wherein saidindolizidine alkaloids have the structure ##STR23## and; (a) A is eitherCH₃ or H, and(b) n is an integer from 1-9, and (c) R is CH₂ OH, CHO, or##STR24## and (d) R₁ and R₂ are alkyl groups both larger than CH₃, or acombination of alkyl groups, one being CH₃ and the other larger thanCH₃.
 17. A method for increasing the force and rate of atrial tissuecontractions comprising contacting substantially pure syntheticindolizidine alkaloids with said atrial tissue in aqueous solutioncontaining 3-6 uM of said indolizidine alkaloids wherein saidindolizidine alkaloids have the structure ##STR25## and (a) A is CH₃, nis 1 and R is CH₂ OH, or(b) A is CH₃, n is 1 and R is CHO, or (c) A isCH₃, n is 1 and R is ##STR26## (d) R₁ and R₂ are alkyl groups bothlarger than CH₃, or a combination of alkyl groups, one being CH₃ and theother larger than CH₃.
 18. A method for increasing the force and rate ofatrial tissue contractions comprising contacting substantially puresynthetic indolizidine alkaloids with said atrial tissue in aqueoussolution containing 3-6 uM of said indolizidine alkaloids wherein saidindolizidine alkaloids have the structure ##STR27## and (a) A is H, n is1 and R is CH₂ OH, or(b) A is H, n is 1 and R is CHO, or (c) A is H, nis 1 and R is ##STR28## and (d) R₁ and R₂ are alkyl groups both largerthan CH₃, or a combination of alkyl groups, one being CH₃ and the otherlarger than CH₃.